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Monday, July 22, 2013

The findings suggest that "global declines in pollinators could have a bigger impact on flowering plants and foods than previously realized," says ecologist Berry Brosi.

By Carol ClarkRemove even one bumblebee species from an ecosystem and the impact
is swift and clear: Their floral “sweethearts” produce significantly fewer
seeds, a new study finds.The study, to be published by the Proceedings of the National Academy of Sciences, focused on the interactions between bumblebees
and larkspur wildflowers in Colorado’s Rocky Mountains. The results show how
reduced competition among pollinators disrupts floral fidelity, or
specialization, among the remaining bees in the system, leading to less
successful plant reproduction.“We found that these wildflowers produce one-third fewer
seeds in the absence of just one bumblebee species,” says Emory University
ecologist Berry Brosi, who led the study. “That’s alarming, and suggests that global
declines in pollinators could have a bigger impact on flowering plants and food
crops than was previously realized.”The National Science Foundation (NSF) funded the study,
co-authored by ecologist Heather Briggs of the University of California-Santa
Cruz.About 90 percent of plants need animals, mostly insects, to
transfer pollen between them so that they can fertilize and reproduce. Bees are
by far the most important pollinators worldwide and have co-evolved with the
floral resources they need for nutrition.During the past decade, however, scientists have reported dramatic
declines in populations of some bee species, sparking research into the
potential impact of such declines.Some studies have indicated that plants can tolerate losing
most pollinator species in an ecosystem as long as other pollinators remain to
take up the slack. Those studies, however, were based on theoretical computer
modeling.

Emory University
ecologist
Berry Brosi led the study.

Brosi and Briggs were curious whether this theoretical
resilience would hold up in real-life scenarios. Their team conducted field experiments
to learn how the removal of a single pollinator species would affect the plant-pollinator
relationship.“Most pollinators visit several plant species over their
lifetime, but often they will display what we call floral fidelity over shorter
time periods,” Brosi explains. “They’ll tend to focus on one plant while it’s
in bloom, then a few weeks later move on to the next species in bloom. You
might think of them as serial monogamists.”Floral fidelity clearly benefits plants, because a
pollinator visit will only lead to plant reproduction when the pollinator is
carrying pollen from the same plant species. “When bees are promiscuous, visiting plants of
more than one species during a single foraging session, they are much less
effective as pollinators,” Briggs says.The experiments were done at the Rocky Mountain Biological Laboratory near Crested Butte, Colorado. Located at 9,500
feet, the facility’s subalpine meadows are too high for honeybees, but they are
buzzing during the summer months with bumblebees. The experiments focused on
the interactions of the insects with larkspurs, dark-purple wildflowers that
are visited by 10 of the of the 11 bumblebee species there.

Watch a video about the Rocky Mountain Biological Laboratory: The study included a series of 20-meter square wildflower
plots. Each was evaluated in a control state, left in its natural
condition, and in a manipulated state, in which bumblebees of just one species had been removed using nets.“We’d literally follow around
the bumblebees as they foraged,” Briggs says, describing how they observed the bee behavior. “It’s challenging because the
bees can fly pretty fast.”Sometimes the researchers could only record between five and
10 movements, while in other cases they could follow the bees to 100 or more
flowers.“Running around after bumblebees in these beautiful
wildflower meadows was one of the most fun parts of the research,” Brosi says. Much
of this “bee team” was made up of Emory undergraduate students, funded by the
college’s Scholarly Inquiry and Research at Emory (SIRE) grants and NSF support
via the Research Experience for Undergraduates (REU) program.The Rocky Mountain Biological Laboratory is exacting about
using non-destructive methodologies so that researchers don’t have a negative
impact on the bumblebee populations. “When we caught bees to remove target
species from the system, or to swab their bodies for pollen, we released them
unharmed when our experiments were over,” Brosi says. “They’re very robust
little creatures.”No researchers were harmed either, he adds. “Stings were very
uncommon during the experiments. Bumblebees are quite gentle on the whole.”Across the steps of the pollination process, from patterns
of bumblebee visits to plants, to picking up pollen, to seed production, the
researchers saw a cascading effect of removing one bee species. While about 78
percent of the bumblebees in the control groups were faithful to a single
species of flower, only 66 percent of the bumblebees in the manipulated groups
showed such floral fidelity. The reduced fidelity in manipulated plots meant
that bees in the manipulated groups carried more different types of pollen on
their bodies than those in the control groups.These changes had direct implications for plant
reproduction: Larkspurs produced about one-third fewer seeds when one of the
bumblebee species was removed, compared to the larkspurs in the control groups.“The small change in the level of competition made the
remaining bees more likely to ‘cheat’ on the larkspur,” Briggs says.While previous research has shown how competition drives
specialization within a species, the bumblebee study is one of the first to
link this mechanism back to the broader functioning of an ecosystem.“Our work shows why biodiversity may be key to conservation
of an entire ecosystem,” Brosi says. “It has the potential to open a whole new set
of studies into the functional implications of interspecies interactions.”